The present invention relates to a ball mill. The ball mill is used to effect a reduction in the size of ore particulate, especially samples of ore obtained from an ore body. In particular, the present invention relates to a ball mill that is capable of milling small samples of ore continuously in a manner that provides information that is usable for the design of a large commercial scale ball mills, and to do so with smaller sample sizes than currently utilized in the industry. The ball mill also provides representative samples of ore suitable for further testing e.g. studies of leaching and concentration of the ore. The ball mill of the present invention will be referred to herein as a semi-autogenous grinding (SAG) mill, to distinguish over ball mills known in the art.
Ore that is mined from the ground, whether in a surface mine or from underground, is obtained in a wide variety of sizes of particulate, varying from relatively small sizes to large chunks of mineralized material. The ore must be reduced to a size of particulate that is suitable for leaching or other separation of metal values from the ore.
A variety of techniques are used in the industry to effect size reduction, examples of which include crushing, rod mill and ball mill grinding, autogenous (AG) grinding and SAG milling. In SAG milling, the ore is crushed in a rotating mill that contains balls. An autogenous mill differs from a SAG mill in that it is operated with no steel or other balls. The balls in SAG milling are usually steel balls. As the mill rotates, the balls are lifted and then dropped onto the ore. The impact causes the ore to be crushed, cracked, broken or otherwise formed into smaller particulate. When the particulate reaches the required size for subsequent processing of the ore, the particulate is removed from the ball mill through discharge ports or grates. Selection of the particulate to be discharged and removed from the system is controlled by the size of the discharge grates, and the use of screens or the like. In this manner, the SAG mill may be operated in a substantially continuous manner, although it is necessary to empty the mill from time to time examine the charge for removal of ore that is not being crushed.
Commercial scale SAG mills are large, and process many tons of ore per hour. It is to be understood that the requirements for a SAG mill will differ depending on the characteristics of the particular body of ore that is to be processed. Moreover, the ore will normally not have the same characteristics throughout the body of ore. For instance, the hardness characteristics of the ore and the concentration of metal values are likely to vary. Some parts of the body of ore may be formed of relatively soft rock compared to other parts of the ore body. Consequently, the design of a commercial scale SAG mill needs to be optimized for efficiency in processing of a particular body of ore. Thus, before a commercial scale SAG mill may be designed and constructed, it is necessary to test the milling characteristics of the ore body, which in turn requires testing of samples from different parts of the ore body. The results obtained are used in the design of the commercial scale SAG mill.
A standard procedure in the industry is to utilize a pilot scale SAG mill having a diameter of six feet. Such a pilot scale SAG mill is used to provide data on flow charts for the ore, and grinding characteristics such as specific energy to achieve the required fineness and product size distribution of the ground material that is representative of and can be used in scale up for the design of a commercial scale SAG mill. However, a pilot scale SAG mill having a diameter of about six feet processes about one ton per hour of ore, and each test must be conducted for several hours in order to obtain data needed for scale-up calculations. Thus, a large quantity of ore is required for any test. As any one sample of ore is not characteristic of the entire ore body, it is necessary to obtain and process numerous samples from the ore body, and many tons of each sample are needed.
The alternative used in the industry is to utilize a SAG mill having a diameter of about one foot. A SAG mill of this small size requires a 2 kg sample of the ore that is run as a batch laboratory test, not as a continuous pilot plant test. As substantially less of each sample of ore is needed, the time and effort to obtain and provide numerous samples of the ore body and the time to process the samples in this small size of SAG mill are significantly reduced. However, the one-foot SAG mill only provides data on ore hardness, the projected energy requirements and the amount and size of fines particles that are produced. This is sufficient data for calculations on the scale up of the size of the SAG mill to a commercial size, when enough data is obtained to define the hardness variability function for the body. However, this test does not provide on-line continuous process data that validates the laboratory work and that clients and investors require to prove that the process will work. It also does not provide material that has been ground, grinding being a major contributor to the milling of ore in a commercial scale ball mill. Meaningful pilot plant tests on SAG ball mill ground ore cannot be obtained. In particular, minimal or no data on the grinding aspects of operation of a commercial SAG mill is obtained. Thus, the designer of the commercial scale SAG mill is forced to make assumptions in the calculations, without actual pilot plant support and with no evidence on whether downstream metallurgical processes will respond in the manner predicted from pilot plant work that does not use the proper grinding process.
In North America, the majority if not all of the metallurgical testing is done at a scale of about 100 to 200 kg per hour, with grinding preparation being done on fine crushed ball mill ground ore. By omitting SAG grinding on this material, the opportunity to make serious process selection mistakes is greatly increased, especially when excess SAG generated fines consume large quantities of expensive reagents. The consequence is that a proposed commercial scale SAG mill has not been properly evaluated and that the process being built may be inefficient.
Pilot plant SAG mills with diameters of approximately six-feet have been the test SAG mills accepted and utilized in the industry for about fifty years. However, a more effective apparatus and method for testing samples of an ore body prior to the design of a commercial scale SAG mill and the following processes, is required.
One aspect of the present invention provides a pilot plant SAG mill comprising a cylindrical outer chamber having flanges at opposed ends, said cylindrical outer chamber having a diameter of 2.5-5.5 feet and a ratio of length to diameter in the range of greater than 1:1, said cylindrical outer chamber containing a removable grinding chamber in the form of a sleeve, longitudinal lifters and a diaphragm, said removable grinding chamber having a ratio of diameter to length in the range of 3:1 to 1:1 and containing a plurality of steel balls not exceeding 15% of the grinding chamber volume, said removable grinding chamber extending partly down the length of the cylindrical outer chamber and having said longitudinal lifters attached to the internal surface of the sleeve, said lifters being capable of lifting steel balls and ore located in the removable grinding chamber during rotation of the cylindrical chambers, said removable grinding chamber having means at one end for receiving particulate ore from a feed hopper and said removable diaphragm at the opposed end, said removable diaphragm having outlet ports therein for discharge of ground particulate ore into the cylindrical outer chamber, said cylindrical outer chamber having discharge ports for discharge of ground particulate from the SAG mill, and a means to rotate the cylindrical outer chamber about a longitudinal axis.
In embodiments, the means to rotate the cylindrical outer chamber is by use of a vari-speed chain or V-belt drive, especially using a pulley or sprocket bolter at the discharge end flange of the cylindrical outer chamber.
Another aspect of the present invention provides a method of testing the milling properties of a particulate ore, comprising:
feeding particulate ore to a pilot plant SAG mill, said SAG mill comprising a cylindrical outer chamber having flanges at opposed ends, said cylindrical outer chamber having a diameter (d) of 2.5-5.5 feet and a ratio of length to diameter in the range of greater than 1:1, said cylindrical outer chamber containing a removable grinding chamber in the form of a sleeve, longitudinal lifters and a diaphragm, said removable grinding chamber having a ratio of diameter to length in the range of 3:1 to 1:1, said removable grinding chamber extending partly down the length of the cylindrical outer chamber and having said longitudinal lifters attached to the internal surface of the sleeve, said lifters being capable of lifting steel balls and ore located in the removable grinding chamber during rotation of the cylindrical chambers, said removable grinding chamber having means at one end for receiving particulate ore from a feed hopper and said removable diaphragm at the opposed end, said removable diaphragm having outlet ports therein for discharge of ground particulate ore into the cylindrical outer chamber, said cylindrical outer chamber having discharge ports for discharge of ground particulate from the SAG mill, and a means to rotate the cylindrical outer chamber about a longitudinal axis, said removable chamber containing a plurality of steel balls having a diameter in the range of 1 to 4 inches, said steel balls occupying not more than 15% of the volume of the removable grinding chamber;
rotating said cylinder about a longitudinal axis at 65-80% of critical speed (Sc), said critical speed in rpm being defined as
Sc=76.63/d, where d is the grinding chamber inside diameter in feet
and
discharging ground particulate ore through said removable diaphragm into said cylindrical grinding chamber, and discharging said ground particulate ore from the cylindrical chamber.
In preferred embodiments of the method, the ground particulate ore is discharged discharge ports e.g. through 5xc3x975 inch discharge ports, in the cylindrical outer chamber, located in sequence in circumferential rows down the length of the cylindrical outer chamber.